In certain situations it is desired to detect a pulsed wave having a predetermined carrier frequency and predetermined time position. The detection process is hindered because the carrier frequency of the pulsed wave has a variable, unpredictable phase and must be detected in the presence of background impulse energy containing the same frequency as the wave. The background energy generally subsists at the same frequency as the duration of the wave carrier frequency pulses.
In particular, in article surveillance systems, a first inductive magnetic field having on and off duty cycle portions is derived by a generator. The generator derives the first magnetic field so it has a predetermined carrier frequency during the on duty cycle portions. An article to be detected for surveillance purposes includes a structure similar to a tuned circuit or a resistance-inductance-capacitance (RLC) circuit that responds to the predetermined carrier frequency of the first magnetic field. The structure is arranged to derive a second inductive magnetic field at a predetermined frequency which is equal to or different slightly from the frequency of the first magnetic field. A receiver responds to the carrier frequency derived from the structure to derive first and second different responses while an article including the structure is in and is not in a detection region magnetically coupled to the receiver and the transmitter.
One type of prior art receiver includes processing circuitry that is enabled when the transmitter or generator on duty cycle has been completed. The processing circuitry responds to the second inductive magnetic field for a predetermined interval. The processing involves filtering the carrier frequency of the second magnetic field by use of a high Q bandpass filter tuned to the carrier frequency. It has been found that this type of time and frequency discrimination does not eliminate false alarms caused by magnetic field impulses. This is because the high Q bandpass filter has a tendency to be rung by impulse type inductive field noise, since the impulse noise is in the bandpass of the filter. The noise impulse excites the filter, causing the filter to ring and derive a wave that has virtually the same frequency, duration and amplitude as a waveform derived at the output of the filter in response to an article containing the structure causing derivation of the second magnetic field.
It is, therefore, accordingly an object of the present invention to provide a new and improved detection apparatus and method for a pulsed wave having a predetermined carrier frequency, variable unpredictable phase and a predetermined time position, wherein the wave is susceptible to being derived in the presence of background energy having either the same frequency as the wave for relatively short intervals or differing frequencies from the wave carrier frequency.
Another object of the invention is to provide a new and improved apparatus for detecting a pulsed unpredictable phase and predetermined time position, and wherein the apparatus is unresponsive to pulsed energy outside of a pass for the detector. Another object of the invention is to provide a new and improved article surveillance system employing a receiver that is synchronized with a carrier wave derived from energy storing structures on an article passing through a region supplied with a pulsed magnetic field, wherein the receiver is: responsive to the energy from the structure, effectively unresponsive to the energy from the field generating means, and is immune to magnetic field impulses.